Structures of Self-Assembled Nonmetal Borates Derived from α,ω-Diaminoalkanes

Abstract

A series of crystalline nonmetal borate compounds was prepared by reactions of linear α,ω-diaminoalkanes, H2N(CH2)nNH2 (n = 5−12), with excess boric acid in aqueous solution. These simple reagents self-assemble in water to form complex ordered structures. The products were characterized by a combination of elemental, thermal, and X-ray diffraction analyses as well as single-crystal X-ray diffraction studies. In each case except for n = 7 and 12, a bis(pentaborate) product of general formula [H3N(CH2)nNH3][B5O6(OH)4]2 (1, 2, 4−7) was obtained in good yield. When n = 12, the product, [H3N(CH2)12NH3][B5O6(OH)4]2·4H2O (8), contained interstitial water. When n = 7, a product of composition [H3N(CH2)7NH3][B8O10(OH)6]·2B(OH)3 (3) was obtained, containing an unusual octaborate anion and interstitial boric acid. The borate anions form hydrogen-bonded supramolecular anionic frameworks that host alkyldiammonium dications. These cations contribute to hydrogen bonding and arrange either extended linearly or folded to varying degrees across channels. This study reveals an interplay between the anionic borate host and the cationic guest in which the nature of the cation controls the borate framework structure, which in turn influences cation conformation.